1. Field of the Invention
This invention relates to electronic circuitry located within a telephone company (telco) network for separating signals of a digital subscriber line (DSL), at relatively high carrier frequencies from a voice telephone signal at the relatively low audio frequency band. More particularly, it relates to an arrangement of circuit cards for this purpose, mounted to a back panel, wherein the cards are provided with facilities for the implementation of test procedures, and for a system in which such cards are used.
2. Prior Art
It is the practice in the transmission of DSL signals with audio signals that the DSL signals include both up- and down-link signals propagating between the telco and a subscriber. The up-link and the down-link signals are provided at different carrier frequencies to enable separation of the signals for communication on their respective channels. The up-link and the down-link signals are communicated by a twisted wire pair along with the audio signals, and signal-splitter circuitry is employed both at the telco and at the subscriber station (customer) to separate the DSL signals from the voice signal to permit separate processing of the DSL signals and the voice signals.
As a convenience in the construction of the signal-splitter circuitry within the telco network there is a practice of sometimes mounting multiple signal-splitting circuitry for each of numerous subscribers on a single circuit card at the telco. Even if only one signal splitting circuit, for a single subscriber, is placed on a card, numerous ones of these cards are mounted to a single back plane which provides for interconnection circuitry among the cards, as well as for connection of the DSL and voice signals to points distant from the telco. Cards can readily be replaced, and additional cards can be mounted to the back plane by use of connectors which facilitate connection and disconnection of individual ones of the cards.
Also provided at the telco is the function of testing any equipment associated with subscriber service (DSL equipment including but not limited to: the telco DSL modem, the subscriber DSL modem, the Public Switched Telephone Network, the signal-splitting circuitry and any wiring which carries the signals). In the testing, it is determined whether there is proper operation of the voice and DSL signals. Part of the testing may be accomplished in a manner which does not interfere with the signals, (for example, monitoring). The testing of the DSL equipment associated with a specific subscriber requires a connection of the DSL equipment to test-signal generators and signal measurement devices (test equipment). The connection of the test equipment to the DSL equipment under test is accomplished by electro-mechanical relays, the relays being preferred over purely electronic switching circuitry, to reduce the hazard of possible destruction from high voltages which may occur in the presence of a lightning strike. Furthermore, the relays can maintain connections even if there is no electrical power.
A major consideration in the design of the signal-splitting circuitry is the amount of space required to house and make connections to the numerous cards mounted to the back plane. By way of example, the relay circuitry itself requires space, this space being in addition to the space required by the signal-splitting cards. In presently available signal-splitting equipment, the equipment employs additional cards containing the relay circuitry, the additional cards being mounted nearby or alongside the signal-splitting cards, to the back plane. A large area of the back-plane is required solely for connections between the relays and signal-splitting circuit cards. As a result, the presently available signal-splitting equipment suffers from the disadvantage that much of the space which might be employed for the signal-splitting cards alone is being used for accommodating the additional relay cards. Therefore, a reduced number of signal-splitting cards can be housed within the equipment of a signal-splitting facility of a telco. In other similar equipment, the relays reside on the same card as the signal-splitting circuitry, preventing the possibility of adding or removing the relays alone without disruption of DSL service.